Pulse generator



Uite

This invention relates to pulse generators.

In my Patent 2,942,190, issued June 21, 1960, there is disclosed a pulse generator designed to produce output voltage pulses having extremely steep leading and trailing edges. In this apparatus, first and second grid controlled thyratrons are provided. The cathode of the first thyratron is connected to an output resistor while the cathode of the second thyratron is coupled directly to a source of negative voltage. The plates of both tubes are connected to a delay line and to a source of positive voltage. When a positive signal is applied to the grid of the first thyratron, the first thyratron conducts producing a voltage across the output resistor. After a predetermined interval a second positive signal is applied-to the grid of the second thyratron causing it to conduct and its plate voltage to drop to a negative value slightly above its cathode potential. Since the plates of the two thyratrons are coupled together, the first thyratron is out off sharply and the output voltage falls rapidly to zero. The drop in theplate voltage of the second thyratron also causes a wave front to travel down the delay line, he reflected at the far end, and return to the input of the line with a magnitude'and polarity large enough to cut off the second thyratron. I

While this, circuit has been found entirely satisfactory for the'generation'of pulses of moderate duration, some difficulties have arisen in obtaining very long or very short pulses. When a pulse of long duration is to be generated, the electrical length of the delay line must be such that the time for a pulse to travel down the line and return is greater than the desired duration of the pulse. This is because a voltage wave front surges down the line when the first thyratron is fired producing I a reflected .wave which will cut off the first tube when it returns, if the first tube has not already been rendered non-conductive by the action of the second thyratron. Longer pulses may be obtained by increasing the electrical length of the delay line but this solution results in expensive and bulky components. In addition, it has been found that unless great care and high quality parts are used in the design of the delay line, ripple voltages will be superimposed on the output voltage regardless of the duration of the pulse due to discontinuities and resultant reflections in the line. These ripple voltages occur because the delay line is connected directly to the plate circuit of the first thyratron while the output pulse is being generated.

Extremely short duration pulses on the order of a fraction of a microsecond are also quite difficult to obtain with the circuit described in the aforementioned copending application due to the finite ionization time of the second thyratron.

Accordingly it is an object of this invention to provide apparatus for generating voltage pulses of any duration.

Another object of the invention is to provide a pulse generator in which the duration of the output pulses is continuously variable and in which the output pulses have rapidly rising and decaying leading and trailing edges.

Still another object of the invention is to provide a pulse generator using a delay line in which the electrical length and other characteristics of the line are not critical.

generator which is small in size, relatively inexpensive,

3,079,514 Patented Feb. 26, 1963 BQQ ripple free, and permits the generation of pulses of long duration.

In the present invention, first and second switching elements are provided each having a cathode, an anode, and a control electrode. The cathode of the first switching element is connected to a voltage reference point through an output impedance while the cathode of the second switching element is connected to a source of negative voltage through a pulse forming network or other energy storage device. The plates of the first and second switching elements are coupled to a source of positive voltage.

Normally both switching elements are essentially nonconducting and, therefore, the voltage across the output impedance is zero. When a first input signal is applied to the control electrode of the first switching element, the first switching element becomes highly conductive and a voltage is produced across the output impedance. After a predetermined interval equal to the desired duration of the output pulse a second input signal is applied to the control electrode of the second switching element. The second switching eiement then becomes conductive, its plate voltage dropping to a negative value thereby reversing the polarity of the voltage across the first switching element causing it to return to its non-conductive state. Application of the second input signal also causes a wave to travel down the pulse forming network, be reflected from its far end, and return to the cathode of the second switching element. This reduces the voltage across the second switching element and switches it from its conductive to its non -conductive state. Since the voltage surge down the pulse forming network is initiated by the second input signal, the electrical length of the network does not afiect the duration of the output pulse. Also, since the pulse forming network is effectively out of the circuit during the'time the output pulse is being produced,

no spurious reflections or other disturbances are generated by the network and no distortion appears in the output voltage waveform.

In a preferred embodiment of the invention the first and second switching elements are comprised of solid state gate controlled rectifiers. As will be explained .in greater detail hereinafter, this type of solid-state rectifier normally has a very high impedance between its Plate and cathode. However, when an apropriate signal is applied to the control electrode, or gate, the plate-to-cathode impedance is abruptly switched from a high to a low value andv the rectifier conducts freely. The rectifier is rendered non-conductive again by briefly reducing the plate-tocathode voltage below the holding value.

The above objects of and the brief introduction to the present invention will be more fully understood and further objects and advantages will become apparent from a study of the following description in connection with the drawings, wherein:

FIG. 1A is a schematic representation of a type PNPN semiconductor;

FIG. 1B is a typical plot of the voltage-current characteristics of the device shown in FlG. 1A;

FIG. 2 is a schematic diagram of the invention; and,

'FIG. 3 depicts waveforms useful in explaining the operation of the circuit of FIG. 2.

In FIG. 1A there is shown a schematic representation of a controlled rectifier, having an anode 10, a cathode 11 and a control, or gate, electrode 12 consisting of an ohmic contact to the center P region. The rectifier consists of three rectifying junctions 13, 14, and 15 which divide regions of P and N type semiconductor material. The impedance between the anode 10 and cathode 11 is initially very high and the rectifier is essentially nonconducting in both directions. However, when current is caused to fiow between the control electrode and the athode by application of a suitable signal to the control electrode 12, the rectifier breaks down at a positive anode-to-cathode voltage greater than a few volts. After breakdown the impedance of the rectifier i very low and the current is essentially limited only by the load. The input signal loses control after breakdown and the rectifier is cut ofi by reducing the anode-to-cathode voltage until the forward current falls below the value required to maintain conduction.

The rectifier may also be caused to conduct Without applying an input signal by increasing the plate-tocathode voltage beyond the forward breakover voltage V This is illustrated by the voltage-current characteristics curve of FIG. 1B. As shown, the forward current I remains constant with increasing forward voltage V until the voltage V is reached. At this point the rectifier impedance suddenly decreases, as indicated by the dashed line, and thereafter the forward current increases sharply with small increases in forward voltage. The magnitude of the forward and reverse currents corresponding to voltages between the peak inverse voltage and the breakover voltage V has been greatly exaggerated in FIG. 1B for increased clarity.

FIG. 2 is a circuit diagram of one embodiment of the invention and FIGS. 3A-3E depict idealized waveforms of the voltages appearing at corresponding points in the circuit of FIG. 2. In the embodiment shown in FIG. 2, a pair of gate controlled rectifiers 20 and 21 V are provided. The cathode 22 of rectifier 20 is connected through a load impedance 23 to ground, while the cathode 24 of rectifier 21 is connected through a pulse forming network 25 to a source of negative voltage -V' The pulse forming network shown comprises inductors 26, 27 and capacitors 28, 29 and is of the type shown in FIG. 6.23 (b), page 203 of the book Pulse Generators, by Glasoe and Lebacqz, published by Mc- Graw-Hill Book Co. Inc, although any other suitable energy storage network may also be used. A shunting resistor 30 is connected between the cathode 24 and the source of negative voltage V The plates 31 and 32 of rectifiers 20 and 21 respectively are coupled to a source of positive voltage +V through a common resistor 33. The control, or gate, electrode 34 of rectifier 20 is coupled to a delay circuit 35 through a current limiting resistor 36, while the control electrode 37 of rectifier 31 is coupled to a delay circuit 38 through a resistor 39. The inputs of delay circuits 35 and 38 are connected 'to the output of a trigger pulse source 40. Delay circuits 35 and 38 may consist, for example, of multivibrators while trigger pulse source 40 may be a blocking oscillator.

When the circuit is first energized, both rectifiers 20 and '21 are non-conductive since the voltage across each is less than the breakover voltage V The application of a trigger voltage from source 40 to delay circuits 35 and 38 produces a first'input signal at control electrode '34 and then, after a predetermined interval T produces "a second input signal at the control electrode 37. The input signal applied to electrode 34 causes rectifier 20 to immediately conduct thereby producing an output voltage (FIG. 3C) across impedance 23. Rectifier 20 continues to conduct even after the pulse is removed, since the voltage between its plate 31 and cathode 22 causes a holding current to exist which is greater than I (FIG. 1B).

After the predetermined interval T has expired, and the second control signal is applied to electrode 37, rectifier 21 will conduct causing the voltage on plates 31 and 32 to decrease abruptly to a value slightly greater than the voltage on the cathode 24 of rectifier 21. With the magnitude of voltage V somewhat greater than that of voltage V and the characteristic impedance of network 25 approximately equal to resistances 23 and 33, the voltage on cathode 24 rises from V to a smaller negative value, as shown in FIG. 4E.

The voltage on plate 31 of rectifier 20 is then slightly below ground (FIG. 3D) and rectifier 20 ceases to conabruptly falls to zero. The transfer of rectifier 21 from its non-conducting to its conducting state also causes a voltage surge down the pulse forming network 25 which is reflected from the other end and returns to the cathode 24 after a time interval T This reflected wavefront causes the cathode of rectifier 21 to assume a positive potential thereby cutting ofi rectifier 21. The network 25 then recharges along a curve 50, cathode 24 ultimately reaching its initial value V as shown in FIG. 3B.

Listed below are typical values for the components used in the circuit of FIG. '2.

Using "the values listed for inductors '26, 27 and capacitors 28, 29 the characteristic impedance of the pulse forming network is 50 ohms and the round trip delay time is about 6 microseconds. Type ZJ-39a controlled rectifiers may be used for rectifiers 20 and 21.

The duration T of the output pulse is determined by the differences in delay time of delay circuits 35 and 38. For pulses of long or moderate duration, delay circuit 35 may be omitted and .the output of the trigger source 40 coupled directly to resistor 36. However, to obtain extremely short pulses, delay circuit 35 is used in conjunction with delay 38 since a short time diiference between the outputs of two networks is more easily obtained than is a short absolute delay from a single delay circuit.

An important feature of the invention is that a pulse generator is provided which is capable of obtaining an output pulse of any desired duration. Also, the trailing edge of the pulse has a very rapid rate of decay thereby providing the sharp cut-off required in many applications. Furthermore, the pulse forming network used to obtain the steep trailing edge cannot affect the waveform of the output pulse,,since the network is only connected to the circuit after the output pulse has been terminated and the first rectifier is open-circuited.

What is claimed is:

l. A pulse generator comprising first and secondswitching elements, each of said switching elements having an anode, a cathode, and a control electrode, the impedance between said anode and cathode being switched from a high value to a low value by the flow of control current from the control electrode to the cathode of said switching element, an outputimpedance connected between the cathode of said first switching element and. a common voltage reference point, a pulse forming net.- work having one end connected to the cathode of said second switching element, impedance means for coupling a positive voltage to the plates of said first and second switching elements, means for coupling a negative voltage to the other end of said pulse forming network, and means for selectively applying first and second control signals to the control electrodes of saidfirst and second switching elements, said first control signal causing said first switching element to conduct thereby producing an output voltage pulse across said output impedance, said second con- 'trol signal causing said second switching element to conduct thereby rendering said first switching element nonconductive and abruptly reducing said output voltage to zero, the interval between the applications of said first and second control signals determining the duration of said output voltage pulse, the reflected pulse produced by said pulse forming network rendering said second switching element non-conductive a predetermined interval after application of said second control signal thereto.

2. A pulse generator as defined in claim 1 wherein said means for selectively applying first and second control signals to the control electrodes of said first and second switching elements comprises delay means, said delay means applying said second control signal to the control electrode of said second switching element a predetermined interval after said first control signal is applied to the control electrode of said first switching element, said predetermined interval being equal to the desired duration of the ouput pulse appearing across said output impedance.

3. A pulse generator comprising first and second type PNPN controlled rectifiers, each of said rectifiers having a plate, a cathode, and a control electrode, an output resistor connected between the cathode of said first rectifier and a common voltage reference point, a pulse forming network having one end connected to the cathode of said second rectifier, a plate circuit resistor connected to the plates of said first and second rectifiers, means responsive to an input trigger pulse for coupling a first control signal to the control electrode of said first rectifier, delay circuit means responsive to said input trigger pulse coupled to the control electrode of said second rectifier, means for coupling a first voltage to said plate circuit resistor, and means for coupling a second voltage to the other end of said pulse forming network.

4. A pulse generator as defined in claim 3 wherein said means responsive to an input trigger pulse for coupling a first control signal to the control electrode of said first rectifier includes second delay circuit means.

References Cited in the file of this patent UNITED STATES PATENTS 2,709,746 Page May 31, 1955 2,883,313 Pankove Apr. 21, 1959 2,895,058 Pankove July 14, 1959 2,952,818 Russell et al Sept. 13, 1960 

1. A PULSE GENERATOR COMPRISING FIRST AND SECOND SWITCHING ELEMENTS, EACH OF SAID SWITCHING ELEMENTS HAVING AN ANODE, A CATHODE, AND A CONTROL ELECTRODE, THE IMPEDANCE BETWEEN SAID ANODE AND CATHODE BEING SWITCHED FROM A HIGH VALUE TO A LOW VALUE BY THE FLOW OF CONTROL CURRENT FROM THE CONTROL ELECTRODE TO THE CATHODE OF SAID SWITCHING ELEMENT, AN OUTPUT IMPEDANCE CONNECTED BETWEEN THE CATHODE OF SAID FIRST SWITCHING ELEMENT AND A COMMON VOLTAGE REFERENCE POINT, A PULSE FORMING NETWORK HAVING ONE END CONNECTED TO THE CATHODE OF SAID SECOND SWITCHING ELEMENT, IMPEDANCE MEANS FOR COUPLING A POSITIVE VOLTAGE TO THE PLATES OF SAID FIRST AND SECOND SWITCHING ELEMENTS, MEANS FOR COUPLING A NEGATIVE VOLTAGE TO THE OTHER END OF SAID PULSE FORMING NETWORK, AND MEANS FOR SELECTIVELY APPLYING FIRST AND SECOND CONTROL SIGNALS TO THE CONTROL ELECTRODES OF SAID FIRST AND SECOND SWITCHING ELEMENTS, SAID FIRST CONTROL SIGNAL CAUSING SAID FIRST SWITCHING ELEMENT TO CONDUCT THEREBY PRODUCING AN OUTPUT VOLTAGE PULSE ACROSS SAID OUTPUT IMPEDANCE, SAID SECOND CONTROL SIGNAL CAUSING SAID SECOND SWITCHING ELEMENT TO CONDUCT THEREBY RENDERING SAID FIRST SWITCHING ELEMENT NONCONDUCTIVE AND ABRUPTLY REDUCING SAID OUTPUT VOLTAGE TO ZERO, THE INTERVAL BETWEEN THE APPLICATIONS OF SAID FIRST AND SECOND CONTROL SIGNALS DETERMINING THE DURATION OF SAID OUTPUT VOLTAGE PULSE, THE REFLECTED PULSE PRODUCED BY SAID PULSE FORMING NETWORK RENDERING SAID SECOND SWITCHING ELEMENT NON-CONDUCTIVE A PREDETERMINED INTERVAL AFTER APPLICATION OF SAID SECOND CONTROL SIGNAL THERETO. 